I mostly agree with your general sentiment that the marine flanks should be made easier if possible and the xeno death ball is always a very hard problem… But, I think you’re missing the key piece of the puzzle here.
Marine power vs xeno power is sort of a math equation where both sides should ideally be roughly equal. So if we’re doing all those things you suggested to empower marines, we should either nerf them elsewhere or buff xenos to compensate.
But in actuality, what keeps happening is that xenos get routine nerfs one after the other, with maybe a couple of very rare small and barely relevant niche buffs per year. Marines, on the other hand get a lot of “new” powercreep, but it’s almost all support stuff - CAS, mortar, Research buffs, now the new Defcon giving them a lot more sustain, the new nuke giving them that “win by default if you stall long enough” button.
So if the marine support side is so strong and would keep getting stronger and the xenos are barely holding on from the nerfcreep, then the basic marine frontline has to be weak and keep getting weaker too.